In many operation processes involving the use of fluids, there often exits a need to pass fluid though a sieve or filter to remove one or more components from a fluid. In addition to removing unwanted components of a fluid, these sieves or filters can also be used to control the rate of fluid flow through a defined space. In many operations, however, conventional filters and sieves are impracticable to use. To this end, fine screens and the like are often utilized as a filter or as a flow regulator. Often, due to the particular requirements of the process, very minute holes are needed to create a filter or flow restriction device. These holes can be extremely small, requiring densities of tens of thousands of holes per square inch.
For example, in satellite fuel systems, components contained within the satellite fuel system may need to be filtered out to create the desired fuel mixture. As the filtering or screening process takes place on the molecular level, the tolerances and accuracy of the hole placement are of critical importance. In this situation, overlap between adjacent holes can cause the total failure of a component because molecules with a diameter greater than a single hole can pass through the enlarged hole of the sieve or filter.
Typically, these devices are made by forming the desired pattern of perforations or holes in a flat or planar sheet of material. This is usually done using a pulsed laser or electron beam. The perforated material is then formed into the desired shape, i.e., a cylinder, and then welded together. Unfortunately, this method suffers from a number of limitations. Initially, the perforated area is not continuous over the entire body of the object because a portion of the surface is taken up by the weld joint. Related to this problem is the fact that the welding process often damages or creates unwanted stress in the perforated component. In addition, since the formed object is initially formed as a flattened sheet, this limits the advantage of increasing the structural strength by employing reinforced bands, ribs, end caps or the like.
Alternatively, instead of creating non-planar components from a linear sheet of material, a component of pre-formed geometry can be machined from solid stock, spun form, or the like, to create a non-planar component without any welds. The non-planar component can then be perforated by the use of a laser or electron beam to create a plurality of holes in the surface of the component. Unfortunately, heretofore, the process of perforating a non-planar object has suffered from a number of limitations.
One particular problem concerns the accuracy of placing the individual holes in the desired location on the component. Often, as a result of the formation process, the holes are located in unintended positions on the non-planar object. This can result in the deleterious overlap of adjacent holes in the object, thereby creating holes that are larger than desired. In addition, when holes are placed too close to one another on the exterior of the object, the web portion residing between adjacent holes is weakened and can lead to structural failure of the component. Moreover, heretofore, it has been economically prohibitive and time consuming to accurately produce high-density holes on non-planar objects. This problem is even a greater concern when high-density configurations are needed that require intricate patterns on the surface of the object. For example, some jobs require that the hole pattern alternate among adjacent rows of holes on the surface of the object. This pattern of perforations is particularly hard to create using traditional techniques.
One approach that has been tried is to move the work piece or the laser head to a pre-determined position, and after motion has stopped, a burst of laser energy is fired to create the hole. The disadvantage of this approach, however, is that a significant amount of time is lost in the starting and stopping motion of either the laser head or the work piece. Using this technique, drill rates exceeding four holes/second can be problematic. Another approach that has been used is to fire the laser at a predetermined pulse rate and move the laser or the object at a predetermined speed. This technique is known as "firing on the fly." The disadvantage of this method is that the exact position of the holes cannot be accurately maintained do to the variances in the laser pulse rate or minor fluctuations in motion of the material and/or the laser head. Variations in the velocity of the work piece and laser head create irregular hole placement on the object. This problem is particularly acute during acceleration and deceleration of the laser head or work piece.
Consequently, there remains a need for a method of accurately perforating a non-planar object using a pulsed beam of radiation. The method would provide for accurate placement of a plurality of holes on a non-planar object. The method would allow operators to make high-density patterns on a multitude of shaped objects without resulting in any hole overlap.